1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device suitably used in a liquid crystal display apparatus, a linear image sensor, a solar cell or the like. The present invention also relates to a method for producing a liquid-crystal display apparatus having such a semiconductor device.
2. Description of the Related Art
In recent years, it has been attempted in a liquid crystal display, an image sensor or the like to incorporate an external driver circuit for driving them on the same substrate of the LCD display or the image sensor. In order to lead such an attempt to success, there has been an increasing need for fabricating a semiconductor device such as a thin film transistor on a transparent insulating substrate. The following technique is one example of those techniques to meet the need: A thin film transistor (hereinafter, referred to as TFT) is fabricated by implanting impurity ions to a semiconductor layer with a gate electrode used as a mask to form n-type or p-type source/drain regions in self-alignment with the gate electrode. Such a technique has been developed because of the advantages associated with achieving a short transistor channel length and high performance.
However, in the aforementioned technique, when an inexpensive glass substrate is used as the substrate, it is necessary that the process be generally performed at 600.degree. or less (preferably 500.degree. C.). However, at such a low temperature, it is difficult to form satisfactory source/drain regions. For example, in the case where impurity ions are implanted to a polycrystalline silicon film, silicon atoms are knocked on and displaced from lattice sites by implanted ions, thereby generating lattice defects in the crystals.
Thus, in order to repair lattice damage and activate impurity ions, annealing for activation is required. Typical examples of annealing for activation include a furnace-annealing method, a lamp-annealing method, a laser-annealing method or the like. The furnace-annealing requires a long period of time, for example, 20 hours at 600.degree. C. As a result, the glass substrate is adversely contracted by heat. In the lamp-annealing method, the wavelength of light generated by a lamp is relatively long, whereby the glass substrate is thermally affected, resulting in cracks, warpage or the like. Thus, these disadvantages make it difficult to practice the lamp-annealing method. On the other hand, in the lesser-annealing method, laser light of short wave-length i used so that only the surface of the semiconductor is heated to a high temperature. Thus, the glass substrate is subjected to substantially no adverse effect. Accordingly, the laser-annealing is suitably performed when a device is to be produced using a glass substrate. However, even in the laser-annealing method, it is difficult to form satisfactory source/drain regions, thus making it difficult to sufficiently prevent junction leakage at a drain edge portion during reverse bias.
FIG. 8 is a graph showing the relationship between a drain current I.sub.D and a gate voltage V.sub.G of a conventional n-type polycrystalline silicon thin film transistor. The conventional polycrystalline silicon thin film transistor is produced by implanting impurity ions to source/drain regions of a semiconductor layer in self-alignment with a gate electrode used as a mask, and then activating the impurity ions by the laser-annealing method. This relationship is obtained in the case where a transistor size is L (length)/ W (width)=8/8 .mu.m. The curve indicated by a broken line shows the relationship where the voltage V.sub.DS between the source and the drain is 1 V. The curve indicated by a solid line shows the relationship where the voltage V.sub.DS between the source and the drain is 14 V. When V.sub.DS becomes larger (V.sub.DS is 14 V), the electric field generated at the drain edge becomes larger. As a result, leakage across the drain junction occurs via the crystal defects in the polycrystalline silicon film, and leakage current (I.sub.D) become larger at the reverse bias (V.sub.0 &lt;0).
As described above, such poor junction characteristics prevents a satisfactory switching device from being realized. For example, in order to fabricate a TFT in a pixel portion of an active matrix liquid crystal display, leakage current is preferably several pico-Amperes (pA) or less. Thus, the TFT characteristics shown in FIG. 8 are insufficient.